The present invention relates to a light-beam scanning apparatus in which a light-beam emitted from a light source is reflected by a rotatable polygon mirror and is then applied to a surface being scanned through a flat field lens, and a scanning is thus operated.
In the light-beam scanning apparatus of this kind, as shown schematically in FIG. 1, there have originally been constituted as that a light-beam emitted from laser 2 to the rotatable polygon mirror 1 is reflected by each of the mirror surfaces of the rotatable polygon mirror 1 rotating in the direction of the arrow and then the light-beam is focused to make a light-spot moving over surface being scanned SP through f.multidot..theta. lens 3 at a constant speed in the scanning direction, namely, the direction of x. However, if the mirror surfaces are not in parallel with each other to the rotating axle of the ratatable polygon mirror 1 and the angles of mirror setting are disordered, that is referred to as a tilt angle error, then the light-spot is not shifted uniformly in the direction y that is at right angles to the scanning direction. For example, if there is a tilt angle error of .DELTA..theta. as shown in FIG. 2, then the shifting of .DELTA.d=2f.multidot..DELTA..theta. in which f represents a focal length of f.multidot..theta. lens 3 is to be caused on surface being scanned SP. Such tilt angle error or the chattering vibration of the rotating axle will cause the pitch of scanning line into disorder, so that some countermeasure must be taken. As for one of such countermeasures, it may be considered that such tilt angle error is minimized by improving accuracy of production, however, there are problems that the steps of production will be increased, the mas-production thereof is hardly feasible and the cost will be expensive even if the production can be made accurately, because the accuracy of the production has in fact been nearly maximized. There has also been so far such a countermeasure as shown in FIG. 3 and FIG. 4 respectively, in which a light-beam is made a one directionally converged ray upon the rotatable polygon mirror 1 in form of a linear beam in parallel to the scanning direction by means of a cylindrical lens 4, and a tilt angle error is then compensated by the optical conjugation of the linear beam and a scanning position with respect to the direction being at right angles to the scanning direction through a cylindrical lens (or a toroidal lens) 5 and the flat field lens 3 both arranged between the rotatable polygon mirror 1 and surface being scanned SP, the countermeasure has been disclosed in U.S. Pat. No. 3,750,189. However, the other problems are newly caused that a uniform spot size is hardly obtainable extending over the entire scanning area in the case of using the cylindrical lens 5 (The reason is that a practical focal distance is shorter when the light-beam is incident diagonally to the flat field lens.), and that in case of using the toroidal lens in place of the cylindrical lens 5 a uniform spot size will be obtainable but the cost of such light-beam scanning apparatus would be expensive because of the expensiveness of such the toroidal lens.
Further, there has also been another conventional example as shown in FIG. 5 and FIG. 6, respectively, wherein flat and nearly parallel light-beams, among which a flat light-beam is shown in FIG. 7, are made an incident rays upon rotatable polygon mirror 1, and cylindrical lens 8 is simply arranged between flat field lens 3 and surface being scanned SP. In this device, there is also a problem that a uniform spot size is hardly obtainable extending over the entire scanning area.
FIG. 13(A) illustrates the state described above. Wherein, the reflecting surfaces of rotatable polygon mirror 11 and middle point M on a surface to be scanned are in the relation of conjugation with respect to direction y in the optical system comprising flat field lens 12 and cylindrical lens 13. When a luminous flux is led in through the optical system so as not to produce a light-beam in direction y between flat field lens 12 and cylindrical lens 13, the trajectory of the light-beam becomes linear Wx in direction x while that is curve Wy in direction y. Consequently, the configuration of each spot on the surface to be scanned will become greatly varied in the diameter thereof in the middle of and at the both ends of each scanning line, as shown in FIG. 13(b). In the state like the above, such an infavorable phenomenon as the lowering of the resolving power at every scanning end is taken place. The problem of this kind may be solved by making use of such a toroidal lens 21 being curved in direction x as shown in FIG. 13(c), but a lens in such configuration is more seriously disadvantageous than a simple cylindrical lens is from the viewpoint of the manufacture processes and there will incidentally cause another defect in the former that the optical system thereof will be relatively expensive.